At present, a discharge lamp used as an automobile headlight, is designed to be lighted at about 35 W in a stable lighting time. Such alighting condition is employed for achieving required total light flux, light distribution, and light emission efficiency, and also securing stable lighting. However, there is an increased market needs for an automobile further improved in fuel efficiency, from a concern for a global environment. Further, electric vehicles and automobiles such as a hybrid car with low environmental load utilizing an electric motor as a power source, are popular and spread, and a reduction of power consumption is further strongly desired for on-vehicle lighting such as a headlight.
When a lighting power input into a lamp is only reduced, the total light flux of the lamp is reduced accordingly and the light emission efficiency is also reduced, and therefore an expected performance can not be secured. Accordingly, development of a new lamp capable of satisfying the market needs even at a low power, is desired.
Regarding an automobile headlight for achieving a stable lighting at a low power of 15 to 30 W, Patent document 1 discloses a technique of solving the above-described problem by optimizing a sealed pressure in the light emitting part and an inner diameter of the light emitting part. However, sufficient light emission efficiency can not be obtained only by conditions provided by this document. Therefore, further improvement is desired.